连接两个汇总查询

Question

I have 2 tables that look like this

users :

id | created_at

payments :

id | created_at

I need a table that is grouped by year and month and contains both number of users and payments

stats :

month | year | users | payments

Where users column contains number of registered users and payments - number of payments. I can get two tables separately, but how can I join them?

select 
    month(created_at) as month, 
    year(created_at) as year, 
    count(*) users
from 
    users 
group by 
    month, year 
having 
    users > 0 
order by 
    year desc, month desc;

select 
    month(created_at) as month, 
    year(created_at) as year, 
    count(*) payments
from 
    payments 
group by 
    month, year 
having 
    payments > 0 
order by 
    year desc, month desc;

Answer 1

I think that's what you're looking for :

select a.*, b.payments from (
    select month(created_at) as month, year(created_at) as year, count(*) users
    from users group by month, year having users > 0 order by year desc, month desc
) a left join (
    select month(created_at) as month, year(created_at) as year, count(*) payments
    from payments group by month, year having payments > 0 order by year desc, month desc
) b on a.month = b.month and b.year = b.year

Answer 2

The comparison to users > 0 and payments > 0 are useless. In addition, order by in subqueries is meaningless.

You can do this with a full join :

select month, year, coalesce(users, 0) as users, coalesce(payments, 0) as payments
from (select month(created_at) as month, year(created_at) as year, 
             count(*) as users
      from users
      group by month, year 
     ) u full join
     (select month(created_at) as month, year(created_at) as year,
             count(*) as payments
      from payments
      group by month, year
     ) p
     using (month, year)
order by year desc, month desc;

If you know you have users and payments for all months (that you care about), you can use an inner join rather than a full join .